This invention relates to a screw grommet.
A screw grommet of the type which comprises a head portion, a barrel portion extended downwardly from the head portion and adapted to be inserted through a fitting hole punched in advance in a panel, and a continuous axial cavity through the head portion and the barrel portion, whereby a screw driven into the axial cavity causes at least a part of the enclosing shell of the barrel portion to be outwardly expanded and pressed against the edge of the fitting hole, enabling the screw grommet to be fixed onto the panel has extensive utility as means for fixation of various parts. Although this screw grommet is usable in various ways, it is generally employed for the purpose of uniting a plurality of panel members or fixing one panel member on the barrel portion and, at the same time, securing another panel member in position by the squeezing force generated between the head of the screw and the head portion of the screw grommet.
It is a requirement of screw grommets of the type described above that the grommet should not rotate together with the screw while the screw is being driven into the axial cavity. If the grommet should rotate with the screw, the screw will not be able to enter the cavity and discharge its function. Consequently, the grommet which has been inserted into the fitting hole in the panel cannot be fastened to the panel.
For preventing such rotation of the grommet with the screw as described above, it has heretofore been suggested to cut key grooves in the edge of the fitting hole punched in the panel member for insertion of the barrel portion of the grommet and to raise corresponding ridges or keys from the outer surface of the barrel portion for thereby accomplishing desired union of the grommet and the panel through engagement of the key grooves with the ridges or keys. This measure, however, proves uneconomical because the punching of the fitting hole necessitates use of a punch of special shape or calls for a two-step operation of first punching a fitting hole and subsequently cutting key grooves in the edge of the fitting hole. Other arrangements intended to give better economy than that described above (such as disclosed in U.S. Pat. Nos. 3,869,958, 3,933,076 and 4,070,945, for example) are based on the principle of preventing the undesired rotation by forming the fitting hole in the shape of a rectangle and producing the barrel portion of the grommet to have a section of matching rectangular shape so that the corners of the matched rectangles keep the grommet from being rotated around its axis.
The aforementioned arrangements which make use of matching rectangels indeed prove to be simpler than any other method. Heretofore, however, the barrel portion of the grommet has been produced with a rectangular section and inserted into the fitting hole punched in a matching rectangular shape solely for the purpose of preventing the undesired rotation and idea that the corners of the matched rectangles ought to be utilized for the additional purpose of enhancing the force with which the grommet is fastened to the panel has never occurred to any person skilled in the art.
Specifically, the barrel portion of the grommet which is caused to expand outwardly by the screw being driven into the axial cavity has been intended strictly for the purpose of establishing thorough contact between the outer wall surface thereof and the edge of the rectangular fitting hole. It was thought that the required engagement of the barrel portion with the edge of a fitting hole possessing definite dimensions of its own would best be obtained by outwardly expanding the barrel portion and consequently pressing it against the edge.
When a rectangular fitting hole is to be formed in a panel, however, it is not possible to punch the corners of the hole perfectly at right angles. Conversely, with the punching machine generally adopted for cutting holes in panels, it has become common practice to cut away the four corners of a rectangular die used for punching rectangular holes for the purpose of lengthening the service life of the punching machine in use. Thus the corners of the fitting hole are inevitably rounded to some extent so that the grommet and the fitting hole are not allowed to come into perfect engagement with each other at their corners. Consequently, there is a possibility that the rotary force generated in the grommet while the screw is driven into the axial cavity of the barrel portion of the grommet will build up to the extent of causing the barrel portion to slip on and rotate inside the edge of the rectangular hole of the panel.
The main object of this invention is to provide a screw grommet which can be secured in the fitting hole of a substantially rectangular shape in a panel so fast that the barrel portion thereof held inside the fitting hole will not be rotated with the screw.